This invention relates to the control of a lift of the kind attached to a back part of a freight truck for transporting freight up into and down from the truck. More particularly, the invention relates to a mechanism and a method of control for the opening and closing of the truck's tail gate.
FIGS. 2A and 2B show a lift of this general type attached to a back part of a freight truck. Such a lift generally makes use of a parallel link mechanism comprising a plurality of cylinders to open and close a freight-receiving table (herein referred to as the tail gate), as well as moving it up and down as a whole.
Explained more in detail with reference to FIGS. 3A, 3B, 3C and 3D, a gate-opening mechanism 30 of such a lift for opening and closing a tail gate 31 (that is, a mechanism for rotating the tail gate 31 around an axis between a closed position and an open position), a tilt cylinder 32 with a piston, a main arm 36 and an attachment bracket 38. The tip of the piston of the tilt cylinder 32 is axially connected to the tail gate 31 through a pin 34, and the other end of the tilt cylinder 32 is axially connected to the bracket 38 through another pin 39. Similarly, one end of the main arm 36 is axially connected to the tail gate 31 through a pin 33, and the other end is axially connected to the bracket 38 through another pin 35. The attachment bracket 38 is welded to the chassis 37 of the truck.
The aforementioned rotary (or tilting) motion of the tail gate 31 is generally effected by means of a pair of left-hand and right-hand tilt cylinders, of which only one is shown in FIGS. 3A, 3B, 3C and 3D. Their motion is controlled by a power unit comprising hydraulic pumps, electric motors for operating these pumps and solenoid valves, as well as an electric control unit.
The tail gate of a kind structured as generally shown in FIGS. 2, 3A, 3B, 3C and 3D is usually designed so as to be parallel to the ground when it is stopped at its opened position. When the truck is parked on the ground which is sloped in the forward or backward direction, therefore, not only is it difficult but also dangerous to load or unload a cargo by using such a tail gate remaining parallel to the sloped ground. In other words, loading and unloading could not be done with high efficiency by using such a lift unless the ground was flat.
The tail gate of a prior art lift is adapted to rotate at a same speed whether it is opening or closing. This means that it starts and stops its motion suddenly, making it likely to damage a fragile cargo. Sudden movements of the tail gate are also a problem to be considered from the point of view of safety to the workers.
Another disadvantage of a prior art lift of this type has been the lengths of the oil pipes between the cylinders and the valves. If there is a damage to a pipe, the loss of pressure inside a cylinder cannot be prevented even if valves are closed, and this could lead to a serious accident.